I entered my direct deposit account and payment date for my federal tax payment. If I have not yet clicked "transmit my return", will the IRS still debit my bank account?

No, you will have to transmit the return and have it ACCEPTED before anything else will happen.

If you have not e-filed then you have not sent anything to the IRS yet; so, no, the IRS is not going to debit your account until sometime after they receive your e-file.   If you do not want to specify a date, you can leave the date blank and it will default to the deadline of April 15.